1. Field of the Invention
The present invention relates to disinfected and/or stabilized biosolids and a novel method of producing disinfected and/or stabilized biosolids, using a combination of iron salts and heat drying.
2. Description of Related Art
The treatment of wastewater treatment plant residuals to create value-added products is fast becoming a reality as a result of public opinion and regulatory concerns. This mandate has given impetus to developing innovative approaches for municipal waste residuals treatment based on possible end uses and recycling modes, such as commercial fertilizer, sediment amender for dredge sediment for wetlands and remediation of lead contaminated soils.
This movement is facilitated by advances in solids processing, including incineration, co-generation of biosolids treatment, heat dried biosolids, advanced alkaline stabilization, innovative chemical or thermophilic processes (which ensure disinfection), stabilization and the destruction/biodegradation of refractory organics. These treated municipal waste residuals may be further treated to produce a value-added product that reduces or eliminates pre-existing or perceived liability.
The method disclosed in the present invention allows these tested value-added products to move beyond the criteria developed in the United States Environmental Protection Agency (EPA) 503 regulations, which originated from research conducted in the 1970s and 1980s.
According to 40 CFR 503, biosolids must be stabilized prior to land application in order to reduce pathogens, odors, potential putrefaction processes, and vector attraction concerns. Over the past ten years, several short-term and long-term stabilization techniques have been developed. Long-term stabilization is obtained when the readily degradable organics are decomposed. Processes currently recognized as capable of obtaining long-term stabilization are limited to the biological processes, such as aerobic digestion, anaerobic digestion, composting, lagoon storage and pile storage. Physical and chemical treatment processes, such as acid treatment, alkaline stabilization, and heat drying, are currently delineated as short-term stabilization processes that only inhibit the putrefaction process. With acid- and alkaline-stabilized biosolids, long-term storage may yield odors and attract pests, and pathogen regrowth can occur. Windrow alkaline composting of these biosolids types can result in long-term stabilization and reduced vector attraction.
The indicators used to determine whether biosolids have been treated adequately to achieve stabilization depend on the method of stabilization used and the grade of the biosolids desired. Treated biosolids are divided into Class A and Class B biosolids on the basis of pathogen reduction as determined by microbiological testing. Other assessment methods are used to demonstrate adequate reductions in vector attraction and odor control, such as the specific oxygen uptake rate (SOUR), percent volatile suspended solids (VSS) reduction, and the carbon dioxide release rates. Limited bench-scale experiments on sludges treated with both acid and alkaline systems have noted biosolids stability, as measured by decreased SOUR and reduced VSS. VSS reductions in the acid system ranged between 56% and 74%, which is comparable to the reduction of degradable organics, yet a total solids reduction was not appreciable due to some precipitation of calcium sulfate.
The respiration procedures involve carbon dioxide respiration and oxygen uptake rates traditionally associated only with biological processes. The use of biochemical oxygen demand (BOD5) as a surrogate for SOUR gives a better assimilation of biological stabilization. BOD5 is a powerful tool in assessing the organic strength of biosolids, indicating the amount of oxygen required for biological degradation. The correlation of the BOD5 with the SOUR measured by respirometry provides an average stability instead of a batch assessment, as shown in FIG. 1. The results of these tests are reported as mg/L oxygen consumed per gram dry weight biosolids per hour. Carbon dioxide respiration and oxygen uptake rates are based on the current protocols used to assess compost stability. The Compost Stability Index is based on the level of microbial activity in a sample, determined by monitoring respiration (mg CO2-C/g compost carbon/day).
Stability is, among all the characteristics of sludge, possibly the most difficult one to measure; it can include the entire set of characteristics making the disposal of sludge acceptable. Although there has yet to be a definition of stability with one to two parameters, several aspects have been agreed upon: odor should not be released, vector attraction reduction should be achieved and pathogen reduction should be achieved as prescribed in the 503 regulations.
The concept is difficult to standardize and generalize because stability is associated with a specific location and disposal method. Stability of sludge is generally linked to the putrefaction potential or the tendency of organic matter to biodegrade. The stabilization process is considered as controlled decomposition of easily degradable organic matter resulting in a significant reduction of volatile solids content, a change of an unpleasant smell, and reduction of pathogens. In a WEF/ASCE report, researchers used the level of volatile solids reduction in a sludge stabilization process as the indicator of its performance. It is also recommended that respirometric methods be used to measure biodegradable organic matter content and the stability of the sludge. Compared to other stabilization degree measurements, few studies have been conducted on the respirometric investigation of sludge stabilization.
Overdosing the sludge with iron raised concerns about iron toxicity towards the receiving environment. A study assessing the transformation of iron species in activated sludge membrane bioreactors showed that, although the total iron concentration in engineered biological systems is much higher (e.g., typically over 100 mM in feed streams to activated sludge treatment plants) than in many natural systems (e.g., concentrations of 0.1e9 nM are typical of marine systems), the concentration of iron in solution and available for biological uptake is likely to be similar since, in both instances, under oxic conditions at circumneutral pH values, the thermodynamically favored ferric iron (Fe(III)) is highly insoluble with the concentration of dissolved inorganic Fe(III) species on the order of 10-11 M. Thus, there is less concern about effluent or supernatant generated from iron-added sludge.
A respirometry method has been developed for measuring the oxygen uptake rate of a given sample, either liquid or solid, in a closed system with ample oxygen supply. This design is also able to measure the biogas production of a sample under anoxic or anaerobic conditions, which are usually maintained by purging the closed system of oxygen with nitrogen gas before testing. Researchers have used biological stability to determine to what extent readily biodegradable organic matter has decomposed. Respirometry can also detect if the biosolids exhibit any toxicity toward certain receiving environments, particularly wetland.
A method to assess biological stability must numerically represent the actual point reached in the process of decomposition through the use of a measurement on a recognized scale of values, which enables the comparison of different decomposition processes. One definition of stability that has been offered is the extent to which readily biodegradable organic matter has decomposed; this definition allows for an analysis of the efficiency of waste treatment. Other biochemical parameters, such as volatile suspended solids (VSS), total and dissolved organic carbon (TOC, DOC) and chemical oxygen demand (COD) have been used to monitor the progress of biological treatments. However, when analyzing heterogeneous materials, these parameters lack precision because of the presence of non-biodegradable volatile or oxidizable materials. The use of biological activity measurements as a measure of biodegradable organic matter content or stability has been widely suggested and experimented upon in recent decades. Aerobic respirometric techniques and methanogenic activity assays have been proposed and seem most appropriate.
In the last decade, most of the respirometric applications were to monitor composting; few looked into biological reactor performance. Several indices derived from actual readings have been created and are applicable to a number of different sample matrices: oxygen uptake rate (OUR), respirometric index (RI), and respiratory quotient (RQ). OUR has traditionally been used in aerobic processes to estimate real-time biological activity, and it is broadly used in the field of wastewater treatment. When the same technique is applied to composting material, OUR is often referred to as the dynamic respiration index (DRI). When DRI is estimated off-line and without continuous aeration, the static respiration index (SRI) can be obtained that determines compost stability. Respiration methods that measure OUR are considered the most reliable to determine biological stability because of the ability of OUR to numerically represent the point when readily biodegradable organic matter has been decomposed to a certain level during a stabilization process.
Respirometry has been used as an indicator for biomass inhibition or stimulation by testing endogenous respiration rates without adding oxygen or substrate. Results indicated that the addition of iron-based chemicals, i.e., ferric chloride and ferrous sulphate, did not stimulate the biomass during dosing. OUR increased when iron(II) was added but not iron(III); however, improved biological treatment was not observed with the addition of iron(II), so stimulation was not indicated.
The most useful indicator of sludge stability is its odor, although odor measurement is notoriously difficult. The utilization of a dynamic olfactometer which can be calibrated to individual odor sensitivity is a commonly used technique to measure odor intensity. The most frequent cause of odor concerns is H2S. The conditions that lead to its production also are beneficial to the growth of other odorous organic compounds. Strong offensive gases associated with raw sludges usually disappear during the first stage of the stabilization process. This is consistent with practical experience since aerobic stabilization usually does not create offensive odors at the treatment plant. High oxygen uptake rates are usually associated with high odor intensity index during the stabilization process.
Recent studies indicate the potential widespread occurrence of estrogenic compounds and other organic wastewater contaminants and their metabolites in the environment. Estrogenic compounds include steroid hormones and their metabolic by-products, oral contraceptives and alkylphenols. Both naturally-occurring estrogenic compounds and estrogen-mimicking compounds may have a role in the disruption of normal endocrine functions. The EPA defines endocrine disrupting chemicals (EDCs) as “exogenous agents that interfere with the production, release, transport, metabolism, binding, action or elimination of the natural hormones in the body responsible for the maintenance of homeostasis and the regulation of developmental processes.” EDCs and other organic wastewater contaminants and their metabolites are continually introduced into the environment, and the long-term effects of continuous, low-level exposure on ecosystems and human health are not well understood. Based upon the recognition of the potential scope of the problem, the possibility of serious effects on the health of populations, the persistence of some endocrine-disrupting agents in the environment and widespread global concerns, the EPA has identified EDC research as one of six high priority topics. Further, the World Health Organization has identified exposure to endocrine disrupting chemicals as an emerging global environmental concern.
A relatively high percentage of estrogen compounds, including estradiol, estrone and estriol, is expected to partition into the biological suspended solids in activated sludge due to their low aqueous solubility and moderately hydrophobic character (log Kow 2.6-4.0). Biosolids treatment processes therefore may represent a significant sink for estrogenic compounds. In a recent study, samples collected at various stages of treatment showed that 51%-67% of the estrogenic activity in the influent wastewater was removed by wastewater and biosolids treatment processes. Approximately 5%-10% of the estrogenic activity was associated with the processed biosolids. An increase in estrogenic activity was reported as treatment progressed in aerobic and anaerobic digestion processes. The degradation of some compounds, such as conjugated hormones, may result in greater estrogenic activity in the products of these processes than in the parent compounds. In another study, an increase in natural estrogen concentrations was reported in the water and sludge phases from a mesophilic anaerobic digester in a municipal sewage treatment plant. Despite these findings, there is little information available regarding the fate of estrogenic compounds during various phases of solids processing such as thickening, aerobic digestion, anaerobic digestion, lime stabilization, chemical conditioning, dewatering, drying and composting. These earlier studies show the potential of increased estrogenic activity in treated biosolids and subsequent risk of entry to the environment. Therefore, the effects of specific sludge treatment processes on estrogenic activity in biosolids must be assessed.
Processes that can disinfect and control vector attraction along with deactivating EDCs and pharmaceuticals and personal care products (PPCPs) will become preferred biosolids treatment processes. In the Ferrate Advanced Alkaline Stabilization/Disinfection process, ferrate reacts with these EDCs and PPCPs. Recently, the oxidation of hormonal estrogens, estrone (E1), 17β-estradiol (E2), and 17α-ethynylestradiol (EE2) by ferrate was studied. The results suggest that hormonal estrogens can be effectively removed by oxidation with ferrate(VI). Complete removal was obtained at a molar ratio of ferrate(VI) to estrogens of about 3.0 in water samples at pH 9.0.
Biosolids derived from municipal wastewater treatment are frequently used for beneficial purposes, such as land application to improve agricultural soils. Organic pollutants can partition into sludge during primary or secondary treatment and reenter the environment during biosolids reuse. Further, the occurrence of EDC contaminants in biosolids applied to land is a growing global environmental concern. An understanding regarding the fate of estrogenic compounds and estrogenic activity during solids processing therefore is important in assessing environmental and human health risks and formulating best management practices.
While certain novel features of this invention shown and described below are pointed out in the annexed claims, the invention is not intended to be limited to the details specified, since a person of ordinary skill in the relevant art will understand that various omissions, modifications, substitutions and changes in the forms and details of the invention illustrated and in its operation may be made without departing in any way from the spirit of the present invention. No feature of the invention is critical or essential unless it is expressly stated as being “critical” or “essential.”